The present invention is generally directed to a lamp control circuit for maintaining the light output intensity of an incandescent lamp or banks of lamps to be controlled constant. The present invention is more specifically directed to a lamp control circuit for regulating the light output intensity of the illuminating incandescent lamps of fiche readers and document recorders.
There are many environments in which it is desirable and often necessary to maintain the light output intensity of illuminating incandescent lamps constant. Such environments include fiche readers wherein an incandescent lamp is utilized for projecting individual fiche film frame images onto a projection screen for reading purposes and document recorders wherein banks of incandescent lamps project illuminating light onto an exposure area in which area a series of documents are transported and recorded on film by a scanning camera. In the fiche reader environment it is desirable to maintain the light output intensity of the illuminating incandescent lamp constant notwithstanding varying power supply voltages so that the fiche images can be easily read. In the document recorder environment it is absolutely essential that the light output intensity of the illuminating incandescent lamp banks be constant as a document is recorded to assure even exposure of the document.
Prior art systems for accomplishing this end have generally included switching type regulators wherein a pulsating DC voltage is switched on and off by a suitable switching device and then filtered using inductance-capacitance combinations. The switching regulators normally include a free running oscillator having a variable duty cycle wherein the duty cycle controls the magnitude of the power supply voltage and wherein the duty cycle is controlled in turn by a feedback voltage arrangement. While such systems have proven generally successful the necessary inductance-capacitance combinations are rather bulky and heavy rendering such systems unsuitable for portable use. Additionally, the inductance-capacitance arrangements are not capable of integrating the illumination energy in the exact same manner as the controlled incandescent lamps rendering such prior art systems not totally reliable.
It is therefore a general object of the present invention to provide a new and improved lamp control circuit for maintaining the light output intensity of an incandescent lamp to be controlled constant.
It is a further object of the present invention to provide a lamp control circuit which maintains the light output intensity of an incandescent lamp constant and which does not require the inductance-capacitance arrangements previously required for such applications.
It is a still further object of the present invention to provide a lamp control circuit which affords improved lamp intensity control over those of the prior art.